Various sorts of methods for fabricating the key top are known. Among them a method called laser processing method is widely adopted as the method for fabricating the key top of the type, in which the display portion emits light.
FIG. 4 is a plan view of a key top fabricated by the prior art laser processing method; FIG. 5 is a cross-sectional view thereof; and FIGS. 6A to 6D are schemes illustrating the fabrication steps by the fabrication method.
In FIGS. 4 and 5, reference numeral 1 indicates globally the key top. On the upper surface of the key top 1, there are formed a first display portion 2 and a second display portion 3, which can be seen from the exterior. The key top 1 stated above consists of a base member 4 made of transparent synthetic resin such as acryl resin, etc., an intermediate layer 5, which is e.g. white, disposed on the surface of the base member 4 and made of light transmitting paint, and an external layer 6, which is e.g. black, disposed on the surface of the intermediate layer 5 except for a part thereof and made of light intercepting paint. The first and the second display portion 2 and 3 are composed of the intermediate layer 5 exposed under the external layer 6. Consequently, at a bright place in the day time, etc., it is possible to confirm the first and the second display portion 2 and 3 by seeing by eye the difference in color between the intermediate layers 5 and the external layer 6, utilizing external light. On the other hand, at a dark place in the night time, in a tunnel, etc., a lamp 7 disposed within the key top 1 is lightened. In this way, it is possible to confirm the first and the second display portion 2 and 3 emitting light at the dark place by using the light passing through only the base member 4 and the intermediate members 5 to reach the exterior.
At the fabrication of the key top 1 constructed as described above, at first, the base member 4 made of acryl resin is formed in a desired shape as indicated in FIG. 6A. Thereafter the intermediate layer 5 is applied on the outer surface of the base member 4, as indicated in FIG. 6B, and an external layer 6 is deposited on the intermediate layer 5 by means of application, sputtering, evaporation, etc., as indicated in FIG. 6C. Further a part of the external layer 6 is irradiated with a light beam 8 emitted by a YAG laser device, as indicated in FIG. 6D. The key top 1 indicated in FIG. 4 is fabricated by removing only the parts of the external layer 6 corresponding to the first and the second display portion 2 and 3 by means of this laser light beam 8. Further, on the whole outer surface of the key top 1, a transparent hard coat layer (not shown in the figure) is deposited at need by means of application, sputtering, evaporation, etc.
However, according to the prior art method for fabricating a key top described above, since the external layer 6 deposited on the whole outer surface of the intermediate layer 5 is removed partially by using a laser light beam 8, the portions of the intermediate layer 5, on which the external layer is removed, constituting the first and the second display portions, 2 and 3, there is a problem that it is difficult to display the two display portions 2 and 3 in different colors to make them multicolored in this way.
That is, if the intermediate layer 5 has a two-layered structure consisting of e.g. a red upper intermediate layer and e.g. a blue lower intermediate layer, which have colors different from each other; the first display portion 2 is formed by removing only the uppermost external layer by means of the laser light beam so as to expose the corresponding part of the upper intermediate layer; and the second display portion 3 is formed by removing the external layer and the upper intermediate layer by means of the laser light beam so as to expose the corresponding part of the lower intermediate layer, theoretically it is possible to form a red first display portion 2 and a blue second display portion 3. However, in practice, if the upper intermediate layer is made of a material, which hardly absorbs the YAG laser light, it is difficult to remove the upper intermediate layer to expose the lower intermediate layer. On the contrary, if the upper intermediate layer is made of a material, which absorbs the YAG laser light, when only the external layer should be removed, the upper intermediate layer is also partly removed. Therefore, in either case, it is difficult to make the display portions multicolored without lowering the display quality.